As the heat wave in East Asia drags on, hospitalizing thousands in Japan and straining power resources in South Korea, a new study by European climate researchers predicts that today's unusually high temperatures may become tomorrow's normal summer weather for many regions across the globe.

Published today in the journal Environmental Research Letters, the paper projects that heat waves will become far more common by 2040, even if humans manage to drastically reduce carbon dioxide emissions before then.

Beyond 2040, however, the possibility of the further increase of heat waves is "strongly dependent" on whether or not emissions are reduced, the study states.

"Still, with reducing CO2 emissions, we can reduce the amount of heat extremes very strongly by the end of the century, and this will avoid impacts to both society and the ecosystems," said lead author Dim Coumou, of the Potsdam Institute for Climate Impact Research in Germany.

"Until that time, regions around the world will have to adapt to more extreme heat waves."

Extreme heat to spread beyond the tropics
The researchers used 29 different climate models, checking their accuracy by showing that they correctly represented extreme temperature trends going back to the 1970s. These models show what percentage of the Earth's land are likely to undergo heat extremes in the future.

Heat waves that took place between the years 2000 and 2012, such as the one that parched crops and killed 82 in the United States last year, have affected about 5 percent of global land area. The researchers found that this percentage will likely double by the end of this decade and quadruple by 2040.

"Humans have emitted already a large amount of CO2 in the past, and this will still heat the climate over the next decades -- this is what we call 'committed warming,'" Coumou said.

At the year 2040, however, the models' predictions of future extremes show a huge divergence depending on the emission scenario.

In a future world where carbon emissions are reduced, the number of heat extremes does not increase beyond the middle of this century. But under a high-emission scenario, the researchers project that extreme heat will spread to about 85 percent of global land area. This is because climate change will see the increase of average temperatures around the world, Coumou said, making hot summer weather even hotter.

It is important to note that what the researchers define as a "heat extreme" changes depending on location. Yearly weather conditions vary much more in the mid-latitudes than in the tropics, so it would take a higher temperature jump in Russia than in Brazil to qualify as extreme.

Most of the increase in heat extremes will take place in the tropics, the models predict, such as in western Africa, Indonesia and South America. But heat extremes are also likely to increase by about 20 percent in Western Europe by 2040, the study states.

And under the high emissions scenario, more than 70 percent of the "extra-tropics," including the Mediterranean, the Middle East, parts of western Europe, central Asia and the United States, will experience extreme heat in the summer months by 2100, the researchers found.

This work reinforces earlier studies that have also predicted an increase in heat wave events (ClimateWire, March 18, 2011).

South Korea and Japan try to adjust to deadly heat
"Societies and especially ecosystems are used to the extremes they have experienced in the past," Coumou said, "but they typically have trouble when they are experiencing extremes outside the historic range."

A preview of these troubles is now taking place in southern China, Japan and South Korea as a high-pressure area stagnates over the region. Over the past several weeks, temperature records have been broken in each of these countries, and earlier this week, blackouts seemed likely as the South Korean government enforced energy-saving measures across the country (ClimateWire, Aug. 13)

China's National Meteorological Center (NMC) continued an "orange alert" for the heat wave Tuesday, the 20th straight day the alert was in place, the nation's official press agency reported.

Although blackouts were avoided in South Korea, 875 people have so far been hospitalized for heat-related illnesses this year, the Chosun Ilbo reported Tuesday, and up to 13 people have died. Also because of the heat, the newspaper said, many South Korean schools have extended their summer vacation period.

The Japan Meteorological Agency predicts that the heat wave will continue into next week. On Tuesday, the agency issued a report stating that 106 of the country's observation stations had logged record-high temperatures. Sunday marked the first time Tokyo's temperatures remained more than 30 degrees Celsius (86 degrees Fahrenheit) for the entire day since observations began in 1987, the report said.

According to Yano Katsunori, second secretary of the environment at the Embassy of Japan in Washington, D.C., almost 10,000 people were taken via ambulance to the hospital over the course of last week, and 17 people died.

For Takashi Kisaka, 36, who works for a local forestry cooperative in southern Japan's Kagoshima prefecture, high temperatures have forced him and his co-workers to start work early in the morning to avoid the afternoon heat.

"We can't work around noon because it's very hot, so we start work at 5 am," Kisaka said in an email. "Everybody [is] saying this year is unusual."

Scientific American is part of Springer Nature, which owns or has commercial relations with thousands of scientific publications (many of them can be found at www.springernature.com/us). Scientific American maintains a strict policy of editorial independence in reporting developments in science to our readers.